1. Field of the Invention
The invention relates to a vehicle transmission system for vehicles such as a tractor or a utility vehicle equipped with a cargo.
2. Related Art
Conventionally, as disclosed in Japanese Laid Open Gazette No. 2000-38042, there is a well-known vehicle including an engine and an axle drivingly connected to each other through a power transmission system consisting of a belt-type continuously variable transmission (hereinafter, “CVT”) and a gear-type sub transmission. Due to the CVT, the power transmission system is advantageous for speed change of the vehicle while continuously (not intermittently) transmitting power from the engine to the axle.
However, the CVT has peculiar disadvantages, such as slipping of the belt when wet, short durability of the belt, and inability of engine braking. Further, the gear-type sub transmission requires an onerous clutch-off operation before its gearshift operation.
To solve the problems, it has been suggested that an alternative multi-speed (i.e., discontinuously variable) transmission, such as a gear transmission, solely replace the conventional combination of the CVT and the gear-type sub transmission. However, there still exists the problem that the alternative multi-speed transmission cannot ensure a proper sufficient traveling performance for a vehicle because disengagement of its clutch for gearshift causes a cut-off of the power transmission to the axle, which may cause unexpected movement of a vehicle traveling on a slope.
Further, some works such as snow-removing by a loader requires the vehicle to frequently repeat switching of forward/backward traveling direction (hereinafter, “forward/backward traveling direction switching”). For example, when the vehicle having traveled forward needs to travel backward, the forward-traveling speed stage is manually or automatically shifted down to the neutral stage, so that the neutral stage must be established before a target backward-traveling speed stage is established, thereby requiring a long time for switching the forward/backward traveling direction of the vehicle. Thus, the vehicle is provided with a reverser which can be operated once for promptly changing the forward/backward traveling direction of the vehicle. However, the conventional reverser includes a forward-traveling clutch to be engaged for forward traveling and a backward-traveling clutch to be engaged for backward traveling. The forward-traveling clutch and the backward-traveling clutch are separated from speed-changing clutches of the gear-type sub transmission for establishing various speed stages, and are provided upstream of the speed-changing clutches. Consequently, the reverser increases the number of clutches requiring costs, spaces (obstruction to minimization), complex fluid ducts, and reduction of maintenancability.
Further, it has been suggested that the gearshift of the multi-speed transmission be automatically controlled corresponding to an accelerator operation. However, if the gearshift timing relative to an actual traveling speed of the vehicle, i.e., an actual rotary speed of the axle, is simply fixed, a vehicle ascending a slope or having a heavy weight may have an engine stop because a shift-up timing occurs too early to ensure acceleration of the vehicle, or a vehicle descending a slope may be unexpectedly accelerated because a shift-down timing occurs too late to prevent acceleration of the vehicle so as to void an engine braking, i.e., cause an inertial gravity force of the descending vehicle to overcome the engine braking force.